1. Field of the Invention
The present invention relates to the field of high voltage vacuum switches and circuit interrupting devices and more particularly to an encapsulated vacuum interrupter with a grounded lower end-cup and drive rod, which allows an actuating mechanism to be mounted in close proximity to the vacuum module.
2. Discussion of Prior Art
Encapsulated vacuum switchgear is used to interrupt and control the flow of power through high voltage distribution circuits. As used here, the term “high voltage” refers to a voltage greater than 1000 volts. The encapsulated vacuum switchgear typically includes a vacuum interrupter encapsulated in an epoxy housing mounted to a cabinet or tank for the operating mechanism. The vacuum interrupter includes a pair of contacts, one stationary and one movable between an open and closed position to open and close the electrical circuit. The movable contact is typically mounted on the end of a drive rod, which moves the movable contact between the open and closed position.
The operating rod typically extends from the vacuum interrupter to engage an actuating mechanism mounted in an external cabinet or tank. The portion of the operating rod extending from the vacuum interrupter is insulated to prevent the flow of high voltage electrical energy from the vacuum interrupter to the actuating mechanism cabinet. The actuating cabinet of the actuating mechanism is usually grounded.
The epoxy housing typically includes an internal cavity for supporting the vacuum interrupter and the operating rod. The shape of the internal cavity must be designed to prevent high voltage electrical energy from bridging the gap between the vacuum interrupter and the actuating mechanism cabinet. The high voltage energy will bridge the gap by either “tracking” along the internal wall of the cavity formed in the housing or by striking the actuating mechanism cabinet directly through the cavity.
Tracking is a phenomena resulting from contamination or moisture forming on the internal cavity walls which allows electrical current to creep along the surface of the internal cavity wall from the high potential of the vacuum interrupter to the grounded actuating mechanism cabinet. The electrical current results in heating which over time will degrade the electrical insulating properties of the epoxy insulation and can result in eventual failure. Tracking can be minimized by increasing the distance that the electrical current must creep to reach the grounded actuating mechanism cabinet. Increasing the distance from the vacuum interrupter to the actuating mechanism frame results in a large switchgear device, which is undesirable as it adds cost and requires more space for installation.
U.S. Pat. Nos. 4,568,804; 5,452,172; 6,747,234; 6,828,521 B2; 6,888,086 B2; 7,488,916 B2 and U.S. Pat Application Publication 2006/0231529 A1 disclose several prior art methods of encapsulation that utilize a cavity. These devices utilize techniques such as insulating foam, convoluted cavity walls or semi-conductive coatings to reduce voltage stress or baffles and insulating plugs to increase the path over which current must track down the walls of the cavity. All of these devices are attempts to deal with the high voltage stresses within the internal cavity and they still result in switchgear that is unnecessarily large and still subject to dielectric breakdown of the cavity over time.
U.S. Pat. Nos. 3,471,669, 4,618,749, 5,206,616 and 7,239,490 B2 disclose devices that that have an actuating mechanism in close proximity or in direct contact with the vacuum interrupter. This renders the actuating mechanism at or near same potential as the end of the vacuum interrupter from which the operating rod exits. Such devices must then be fully encapsulated and protected by a grounded metal casing or mounted within another device that has a grounded metal casing to protect the operator from exposure to high voltage. Another device which actually has an exposed electrically hot mechanism housing is demonstrated by U.S. Pat. Nos. 6,753,493 B2 and 6,794,596 B2. This device is meant to be operated in a manner similar to an expulsion fuse and therefore must always be operated by an insulated hook stick in order to prevent the operator from contacting high voltage.
U.S. Pat. No. 6,723,940 B1 discloses a device that utilizes SF6 gas within the internal cavity and depends on the fact that SF6 gas has a higher dielectric strength than air to reduce the size of the internal cavity. However, SF6 gas insulated switchgear can leak over time and SF6 gas has been know to adversely affect the environment as it can affect the ozone layer.
Another interrupting device uses an external air insulated drive rod to eliminate the cavity as demonstrated in U.S. Pat. No. 6,946,614 B2. This design is more complex than those that utilize an internal cavity and quite large due to the clearances required in air.
Still another device utilizes a vacuum interrupter with an internal ceramic contact rod to operate a cantilever beam moving contact and electrically isolate the lower end cup of the vacuum interrupter. This type of device, which is disclosed by U.S. Pat. Nos. 3,178,542 and 5,387,772 does eliminate the need for the internal cavity in the encapsulation and does result in a more compact switchgear unit. However, the ceramic contact rod is exposed to the impact stresses created during closing and the tensile loads created when breaking contact welds upon opening and is therefore subject to breakage.
While the aforementioned prior art arrangements may be suitable for their intended use in accordance with their respective defined applications, as discussed hereinbefore, it would be desirable to provide an encapsulated vacuum interrupter with a grounded lower end-cup and actuating rod.